The present invention generally relates to solid-state x-ray detectors. More particularly, the present invention relates to asynchronous calibration of solid-state x-ray detectors to reduce image artifacts.
Solid-state x-ray detectors have been proposed that comprise a two dimensional array of 1,000 to 4,000 detector elements in each dimension (x,y). Each detector element comprises a photo detector that detects and stores charge representative of an amount of radiation incident on the detector element. Each detector element further includes a thin film transistor (TFT) connected to the photo diode and operated as a switch to enable and disable read out of the charge stored on the photo diode. Each detector element ultimately produces an electrical signal that corresponds to the brightness of a picture element in the x-ray image projected onto the detector. The signal from each detector element is read out individually and digitized for further image processing, storage and display.
The solid-state detector may be used in a variety of x-ray medical imaging applications. Some examples of applications where solid-state detectors are used include angiographic procedures like rotational angiography and bolus chase.
During rotational angiography, a gantry supports an x-ray source on one side of a patient and a detector on the opposite side of the patient. The gantry rotates the x-ray source and the detector around the patient. At predetermined positions during the rotation, the x-ray source emits x-rays through the patient and the x-rays impinge upon the surface of the detector.
In order to associate an acquired image with a particular angular position relative to the patient, the detector is synchronized with the angular position of the gantry. For example, if images are to be acquired with every two degree rotation around the patient, the x-ray source and detector are activated with every two degree rotation of the gantry.
Because some tissues are relatively transparent to x-rays, the patient is sometimes injected with a dye that absorbs x-rays. The x-ray image acquired while the patient is injected with the dye is commonly referred to as a contrast image or an opacified image. Absorption of the x-rays by the dye makes the tissues containing the dye appear darker on the resulting x-ray image.
To further highlight the tissues containing the dye, another corresponding x-ray image taken without the dye, commonly referred to as a mask image, may be subtracted from the contrast image. Subtraction of the mask image from the contrast image results in a combined image with most of the non-dyed material removed. Such contrast and mask images are frequently used to optimize views of vascular structures, stenoses, aneurysms, tumors, and neurovascularization.
Because of physical variations between detector elements, some pixels on a contrast image or a mask image are brighter or dimmer than neighboring pixels despite being subjected to the same level of x-ray exposure. To compensate for differences in intensity between neighboring pixel elements, an offset image may be obtained.
The offset image is typically acquired by exposing the solid-state detector to a uniform level of x-ray exposure or no x-ray exposure. The offset image is then subtracted from the mask and contrast images to cancel or “zero out” some of the effects on image quality that result from physical variations between detector elements.
One of the effects arising from physical differences between detector elements is a change in pixel intensity as the time interval, or frame interval, between image acquisitions is varied. For example, the detector elements may contain residual charge from previous acquisitions and the rate of decay of the residual charge on individual detector elements may vary due to physical variations between the detector elements. Variations in decay rate of residual charge between detector elements may result in differences in the level of charge stored by the detector elements at particular moments in time. Consequently, the level of charge stored by the individual detector elements during an image acquisition may be affected by the time that has elapsed since the last image acquisition.
Because the time interval between image acquisitions from the solid-state detector affects intensity values read from the detector elements, offset images are typically acquired with frame intervals that are identical to frame intervals of corresponding contrast and mask images. The offset images are then subtracted from the contrast and mask images to cancel some of the effects of physical differences between detector elements.
Typically, a set of offset images is acquired before mask images and contrast images are obtained. The time intervals between acquisitions of the offset images are varied to match the potential frame intervals of subsequently acquired mask and contrast images. The offset images are then saved as a set of offset images with each offset image representing a particular frame interval.
After the offset images have been obtained, a first acquisition run is performed where x-ray images are obtained of a patient before dye has been injected into the patient. During the first acquisition run, the gantry will rotate the x-ray source and detector around the patient. Images are acquired at predetermined positions as the x-ray source and detector rotate around the patient.
Next, the patient is injected with dye and the gantry again rotates the x-ray source and detector around the patient. A set of contrast images is acquired at the same positions as the mask images.
Offset images are then selected from the set of offset images based upon the frame intervals of the mask and contrast images. The offset images are then subtracted from the corresponding mask and contrast images to produce offset corrected mask and contrast images that compensate for some of the variation in pixel intensity resulting from physical differences between photodetector elements. Corresponding offset corrected mask and contrast images may then be combined to produce a combined image that highlights the tissues containing the dye.
The mask and contrast runs are performed during high-speed rotation of the gantry. During the mask and contrast runs, the gantry may go through distinct phases of acceleration, constant speed, and deceleration. Because the gantry rotates at high speeds and the speed of the gantry varies during the acquisition runs, it is difficult to synchronize motion of the gantry with the detector reads. High speed and variable rotation of the gantry also makes it difficult to reproduce image acquisitions at the identical positions and frame intervals of prior image acquisition runs. Consequently, it is difficult to acquire mask and contrast images at identical positions in separate runs and it is difficult to acquire offset, mask, and contrast images with identical frame intervals.
Another type of imaging procedure that may involve acquiring offset, mask, and contrast images is a bolus chase procedure. During a bolus chase procedure, a patient is positioned on a movable platform that slides back and forth between an x-ray source and a detector. The patient is injected with a contrast agent. As the contrast agent flows through the circulatory system of the patient, the patient is incrementally moved between the x-ray source and the detector to follow the progression of the contrast agent as it passes through the circulatory system of the patient. Images of the patient and the progression of the contrast are taken at a predetermined interval of patient movement. For example, an image may be acquired every 10 centimeters as the patient slides between the x-ray source and detector.
The rate of patient movement between the x-ray source and the detector depends on the rate at which the contrast flows through the circulatory system of the patient. Because the contrast may flow through different portions of the patient at different rates, the rate of movement of the patient between the x-ray source and the detector may vary accordingly. Consequently, the time intervals between acquisitions of the x-ray images may vary because the time for a patient to move a predetermined interval may vary.
Often, a radiologist or technician is in control of the table speed and is present to view images in real time. Because the radiologist or technician may be diagnosing a patient during the procedure, offset images are used to reduce undesirable pixel variations in images displayed during the procedure in order to provide a more accurate and immediate diagnosis. Thus, for a bolus chase procedure, the offset images are typically acquired prior to acquisition of contrast or mask images so the contrast and mask images may be offset corrected and displayed during the procedure.
However, because the offset images are acquired prior to the x-ray images, the various frame intervals of the x-ray images may not be known when the offset images are acquired. Consequently, the frame intervals of the offset images may not match-up with the frame intervals of the x-ray images obtained during a bolus chase procedure.
As presented earlier, variations in the time interval between image acquisitions may produce variations in pixel intensities. Thus, an offset image representing one frame interval may have different pixel intensity values than another offset image representing another frame interval despite being subjected to the same level of x-ray exposure. Consequently, an x-ray image obtained at a particular frame interval following the last x-ray image acquisition should be offset corrected with an offset image representing the same particular frame interval. Otherwise, if the x-ray image is offset corrected with an offset image representing a different frame interval, the resulting asynchronous combination may still include the effects on pixel intensity caused by variations in frame interval. Variations in pixel intensity in an offset corrected image resulting from an asynchronous combination of images obtained at different frame intervals may appear in final images as temporal artifacts.
Thus, it may be highly desirable to have a system that determines the effect of variations in frame intervals upon the level of charge stored by photodetector elements. It may also be desirable to have a system that determines which pixels show an unacceptable risk for becoming a temporal artifact in a final image because of real time variations in frame intervals and compensates for temporal artifact causing pixels to produce a final image with less image artifacts.